Some apps that show mileage between cities are City Distance, AtoB Distance Calculator Free and US Cities Distance. City Distance is available for Android devices, and AtoB Distance Calculator Free and US Cities distance are available for Apple devices.
The City Distance app is free to download from the Google Play store. The app calculates air and ground distances between two cities from all over the world, and receives a rating of 4.3 out of five stars in the Google Play store, as of July 2015.
AtoB Distance Calculator Free is available for Apple devices with iOS 6.0 and later. The app calculates flight and route mileage for between two and 10 distances at a time. It's available in English, Spanish, German, Italian and Portuguese.